1. Field of the Invention
This invention relates to an improved method for hardening gelatin, and more particularly, to a method for quickly hardening gelatin used in a silver halide photosensitive material. The following description assumes photosensitive materials as typical examples of the application of gelatin.
2. Description of the Prior Art
Gelatin is used as a binder for various photographic light-sensitive materials such as a silver halide emulsion layer, an emulsion protective layer, a filter layer, an interlayer, an anti-halation layer, a backing layer, a film base subbing layer and a baryta layer, and all these layers contain gelatin as a primary component.
These gelatin-containing photosensitive materials are treated with various aqueous solutions having different pHs or temperatures. Those layers which contain gelatin yet to be treated with a hardener depend primarily upon the physical properties of gelatin and are low in water resistance. In aqueous solution, they swell excessively and become very weak in mechanical strength, and in extreme cases, the gelatin layer may dissolve out in an aqueous solution having a temperature higher than 30.degree. C. or in a strong alkaline aqueous solution. These defects are fatal to the use of the gelatin layer as a constituent of a photographic light-sensitive material.
Many compounds have been known to be effective in hardening gelatin to provide a gelatin layer having high resistance to water, heat and physical injury. These compounds are so called "gelatin hardeners" and conventionally used in the production of photographic light-sensitive materials. Illustrative gelatin hardeners include aldehyde compounds such as formaldehyde and glutaraldehyde; compounds having reactive halogen as described in U.S. Pat. No. 3,635,718; compounds having a reactive ethylenically unsaturated bond as described in U.S. Pat. No. 3,635,718; aziridine compounds as described in U.S. Pat. No. 3,017,280; epoxy compounds as described in U.S. Pat. No. 3,091,537; halogenocarboxyaldehydes such as mucochromic acid; dioxanes such as dihydroxydioxane and dichlorodioxane; vinyl sulfones as described in U.S. Pat. Nos. 3,642,486 and 3,687,707; vinyl sulfone precursors as described in U.S. Pat. No. 3,841,872; keto-vinyls as described in U.S. Pat. No. 3,640,720; or inorganic hardeners such as chrome alum and zirconium sulfate.
However, these known gelatin hardeners are defective in one way or another: some of them do not exhibit adequate hardening effect when used in a photographic light-sensitive material; others require a long time for hardening gelatin; still others are made of compounds difficult to synthesize and cannot be synthesized in great quantities; other hardeners are unstable and do not keep long; some smell so bad that their production efficiency is very low; still others are harmful to the human body.
Hardeners providing a particularly high hardening rate are required to have high reactivity and unavoidably they have several defects; for one thing, due to their high reactivity, they are unstable and have a tendency to decompose during their synthesis and for this reason, they are difficult to synthesize in great quantities. For another, highly reactive hardeners easily react with moisture in air and do not keep long. They also decompose rapidly in an aqueous solution or aqueous dispersion in which they are used, and this reduces the amount of the hardener effective for the reaction of hardening and the desired degree of hardening may not be obtained. In addition, such hardeners, their starting materials, and intermediates seem to have harmful effects on the human body, such as carcinogenicity and skin-irritating effect.
But on the other hand, the technology for achieving rapid hardening of the gelatin-containing layer of a photographic light-sensitive material is important to the photographic industry. It is known that the degree to which the gelatin-containing layer is hardened often presents a critical effect on the photographic characteristics of that layer. Therefore, in the photographic industry, light-sensitive materials can be sold to the consumer only after they went through the hardening reaction to provide stable photographic characteristics. This means the manufacturer of photosensitive materials must keep their product in stock until it is completely hardened. The cost of storage is very high, and the longer the time required for hardening, the greater the storage cost. Some manufacturers have tried to increase the hardening rate of photosensitive materials on storage by some means such as heating or humidifying. But such treatment has not been altogether satisfactory because it adversely affects the photographic characteristics of the photosensitive material on storage, or blocking occurs in the roll of photosensitive material. Thus, although an effective method for rapidly hardening gelatin has long been desired in the industry it has been difficult to meet this demand by modifying a gelatin hardener per se.